1. Field of the Invention
This invention relates to proximate surveillance systems utilizing magnetic markers and to a coplanar antenna system for use therein. More particularly, the invention provides a coplanar antenna system that enhances the sensitivity and reliability of the proximate surveillance system with which it is associated.
2. Description of the Prior Art
One of the problems with proximate (less than 1 ft.) surveillance systems employed to detect magnetic markers is the presence of null or dead portions created within the interrogation zone by the orientation dependence of the antenna on the marker. Surveillance systems utilizing magnetic markers differ from electronic or microwave systems in that the frequencies of the signals used to interrogate the magnetic markers are relatively low (below 300 KHZ). These low frequencies require a linear antenna of impractical lengths (more than 500 M); thus alternate antennae configurations are required, such as loops. When applied to proximate surveillance systems, these alternate antennae configurations develop dead portions in which magnetic markers may escape detection. A dead zone is the result of improper orientation of magnetic fields generated by the antenna relative to the orientation of a magnetic marker. At remote detection distances (greater than 1 ft.), the orientation dependence of this antenna on the marker is decreased and the creation of dead portions within the interrogation zone is minimized. Surveillance systems utilizing magnetic markers are presently designed to operate at remote detection distances. Optimum system effectiveness is therein achieved by placing the interrogating and receiving means of the antenna system on opposite sides of the interrogation zone. Such an antenna configuration is unsuitable for a proximate surveillance system, wherein a flat coplanar antenna arrangement is more useful.
One of the problems encountered by coplanar antenna systems, in which the receiving and interrogating means are closely coupled, is known as the transformer effect. The close coupling between the receiving and interrogating means causes signals generated by the interrogating means to be altered by opposing signals induced in the receiving means, disabling the interrogation and detection functions. In an effort to solve this problem, it has been known in the art to form the receiving means as a "figure 8" which, upon exposure to the interrogating means, produces a zero resultant induced voltage in the receiving means. This solution causes a second problem, that is, a dead zone is created in a plane perpendicular to and in the center of the receiving means. To avoid this second problem it has also been known in the art that the interrogating means can be formed as a FIG. 8 and the receiving means be formed as a single loop centered about the interrogating means. This reduces the transformer effect but does not substantially reduce the orientation dependence of the antenna on the marker. One way known in the art to minimize orientation problems when using figure 8's for the interrogating means is to have two sets of figure 8's positioned perpendicular to each other in parallel planes. By having each FIG. 8 alternate interrogations, a magnetic field can be generated that alternates orientation by 90.degree.. But this method has a serious drawback. The transformer effect between the two interrogating figure 8's reduces the actual field experienced by the marker, thereby reducing the system's sensitivity. Still another method known in the art for reducing the transformer effect involves an arrangement in which the same loop is used for each of the interrogating means and the receiving means. With this arrangement, the interrogating and receiving means are connected and disconnected alternately. A drawback of the single loop arrangment is the difficulty of isolating the receiving means from the high power extant during the connect and disconnect of the interrogating means. As a result, proximate surveillance systems of the type described have heretofore had sensitivities insufficient to afford the high reliability required for commercial applications.